As is known, wireless communication systems include a plurality of wireless communication devices and wireless infrastructure devices. The wireless communication devices, which may be radios, cellular telephones, stations coupled to personal computers, laptops, personal digital assistants, et cetera, communicate with each other via wireless communication channels that are administered by the wireless infrastructure devices. Such wireless infrastructure devices include base stations (e.g., for cellular wireless communication systems), access points (e.g., for wireless local area networks), system controllers, system administrators, et cetera. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or multiple channels (e.g., one or more of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel or channels. For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel, or channels. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the internet and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, a receiver receives RF signals, demodulates the RF carrier frequency from the RF signals to produce baseband signals, and demodulates the baseband signals in accordance with a particular wireless communication standard to recapture the transmitted data. A radio receiver is known to include a low noise amplifier, one or more intermediate frequency stages, filters and a receiver baseband processor. The low noise amplifier amplifies radio frequency (RF) signals received via an antenna and provides the amplified RF signals to the one or more intermediate frequency stages. The one or more intermediate frequency stages mixes the amplified RF signal with one or more local oscillations to produce a receive baseband signal. The receiver baseband processor, in accordance with a particular wireless communication standard, decodes and/or demodulates the baseband signals to recapture data therefrom.
As is also known, the transmitter converts data into RF signals by modulating the data to produce baseband signals and mixing the baseband signals with an RF carrier to produce RF signals. The radio transmitter includes a baseband processor, one or more intermediate frequency stages, filters, and a power amplifier coupled to an antenna. The baseband processor encodes and/or modulates, in accordance with a wireless communication standard such as IEEE 802.11a, IEEE802.11b, Bluetooth, Global System for Mobile communications (GSM), Advanced Mobile Phone Service (AMPS), et cetera, to produce baseband signals. The baseband processor produces an outbound baseband signal at a given processing rate. Typically, the processing rate of the transmitting baseband processor is synchronized with the transmitting local oscillation or oscillations and is a fraction of the local oscillation, or oscillations. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce a radio frequency signal. The filter filters the radio frequency signal to remove unwanted frequency components and the power amplifier amplifies the filtered radio frequency signal prior to transmission via the antenna.
Further, data transmissions are serial streams of data, but within a network component (e.g., switch, relay, bridge, gateway, et cetera) the data is processed in parallel. It is a function of the transceiver within each communication device or network component to convert data from a serial to a parallel form, or vice-versa. In general, the transmitter converts parallel data into serial data and sources the serial data onto a communications link. A receiver receives serial data via a communications link and converts it into parallel data.
However, multi-channel transmission systems suffer from signal degradation and data corruption when simultaneously transmitting on channels originating from antennas within reception range of the receiver for which they are intended. This is because currently existing multi-channel systems do not provide for the synchronization of contemporaneously transmitted signals along two or more of their channels. As shown in FIG. 1, each burst of data 2 within a transmitted signal comprises multiple symbols 4 carrying a portion of the data making up the data burst. Each symbol comprises a data signal having a fixed amplitude and phase for the symbol, and a cyclic prefix (known as a guard interval in IEEE 802.11a). The cyclic prefix is essentially a portion of the symbol data repeated prior to the transmission of the data. The end portion of a symbol is copied and repeated within the cyclic prefix for the corresponding symbol. The cyclic prefix is used to allow the noise injected due to the discontinuity from symbol to symbol to settle before the data is actually sampled. As shown in FIG. 1, between the data of one symbol and the cyclic prefix of another symbol, a discontinuity results since the transmitted analog baseband signal is changing from one encoded data set to another.
Unsynchronized signals transmitted contemporaneously from antennas in range of the receiver will thus interfere with one another if the guard intervals of the symbols of one signal are not aligned with the guard intervals of the symbols of the other signal. The interference occurs because the non-aligned noise injected due to the discontinuity from symbol to symbol of one signal will disturb the data portion of the symbols on the other signal, and vice versa. This is illustrated in FIG. 2, which shows an analogous case to that of FIG. 1 for contemporaneously transmitted signals 5 and 6.
Therefore, a need exists for a method and apparatus for synchronizing signal transmission along multiple channels that can reduce or eliminate the problems associated with the prior art.